Teensyduino 1.41 supports Arduino versions 1.0.6 and 1.6.5-r5 and 1.8.1 and 1.8.2 and 1.8.3 and 1.8.4 and 1.8.5.
Future versions of Teensyduino will drop support for Arduino 1.8.2, 1.8.3, & 1.8.4.
On Linux, PJRC tests X86 on Ubuntu and ARM on Raspbian. Other distros may work, but are not supported.

Install Step 1: Download & Extract Arduino

First, you must download the
Arduino Software.
Remember the location where you extracted the files.

Install Step 2: (Linux only) Install udev Rules

Install Step 2: (Macintosh only) Pass Internet Download Question

You must run Arduino at least once before adding Teensyduino, to confirm you
wish to run a program downloaded from the Internet.

If Teensyduino modifies your copy of Arduino before you confirm, recent
versions of OS-X may not allow you to run Arduino, or even tell you Arduino
will damange your computer. Simply run Arduino first to clear the question
about running a program downloaded from the Internet, before adding Teensyduino!

Install Step 2: (Windows Only) Anti-Virus/Malware Exception

Windows Defender,
McAfee and other
anti-virus or anti-malware program can interfere with installation. They
also can cause horribly slow performance when Arduino compiles code, if
"Real Time Scanning" is enabled.

Install Step 3: Run Teensyduino Installer

The Teensyduino installer adds the necessary support files to Arduino.
Your copy of
Arduino must be one of the supported versions
listed on this first screen.

On Windows, this driver allows to you access the serial device type.
This step is not necessary and does not appear on Mac OS-X and Linux.
Windows 10 has the proper driver, which is automatically detected.

Select the location where you extracted the Arduino Software.
On Windows, the location will default to the location where
Arduino's installer places the software.

The Next button will only activate when
a supported version of the Arduino Software is found.

If the Next button will not activate, click the "?" button for
more information.

Teensyduino can automatically install many libraries that are tested
and verified to work with Teensy. Usually it's best to allow the
installer to add them all.

The installer will copy all the necessary files into your Arduino
Software, when you click the "Install" button.

When installation is finished, you will see this final screen. Just
click Done to quit the installer.

When using Arduino, be sure to select Teensy in the Tools > Boards
menu. Arduino's File > Examples menu automatically changes to
show the libraries and examples for the selected board.

Fix initialization of local static vars from non-const data on Teensy 3.x

Make all Teensy 3.1 HardwareSerial functions virtual (fixes issue with pointers to objects)

Add extra zero to USB serial number, works around a Macintosh CDC driver bug
If previous versions have been used, Windows will detect a "new" device, "install" the driver again, and assign a new COM port number.

Version 1.15

Teensy Loader now recognizes ARM elf format, will not accidentally program Teensy 3.0 code to a pre-3.0 board.

Teensy Loader version number is now in sync with Teensyduino version.

Fix stall when connected to a Raspberry Pi that reboots

Add float number support to String

Add String remove()

String now formats unsigned char as a number, same as Print.

Add AVR pin number constants

Added open source license to Teensy3 core library files

Updated Firmata library

Fix USB product name customization header

Reduce USB memory usage

Emulate AVR's TWBR register on Teensy 3.0

Fix single-quote key for USB Keyboard on German and French Canadian layouts

Version 1.14

Updated Time library. "atime_t" on Teensy 3.0 changed back to "time_t". Date strings fixed on Teensy 3.0. NTP and GPS sync examples updated for Arduino 1.0.4. Many other fixes and improvements to the Time library examples.

Added DS1307RTC and TimeAlarms libraries (companions to Time library).

DmxSimple library support for Teensy 2.0 and 3.0 (previously only Teensy++ 2.0 worked)

Fixed FastSPI compile problem on Teensy 2.0 and Teensy++ 2.0.

Update MIDI library to version 3.2.

Fixed error highlighting when using Teensy 3.0.

Improved recompile speed when Arduino is installed in a location with space in the pathname.

Improved automatic initialization of the Teensy 3.0 RTC, when a 32.768 kHz crystal is present.

Fix SD + Ethernet library compatibility on Teensy 3.0.

All 4 IntervalTimer objects usable on Teensy 3.0.

Fix off-by-one bug in delay() on Teensy 3.0.

Enable I/O after wakeup from lowest power stop mode on Teensy 3.0.

Fix analogInputToDigitalPin() on Teensy++ 2.0.

Added OSC (Open Sound Control) library

Updated VirtualWire library to 1.15 and support Teensy 3.0

Updated SoftPWM library to 1.5 and support Teensy 3.0

Updated XBee library to 0.4.

Added partial SoftwareSerial for Teensy 3.0. If the pins match one of the real serial ports, hardware serial is used. This allows use of libraries that are hard-coded for SoftwareSerial. If non-serial pins are specified, only transmit works. Software-based receive is not yet supported (but planned for a future release)

Version 1.13

Support for Arduino 1.0.4

Fix Serial.write(0)

Turkish keyboard layout (Teensy 3.0)

Fix while(!Serial)

Fixes for ST7565 library (Teensy 3.0)

Add OctoWS2811 library

Update libraries: Keypad, Metro, OneWire, Ping, ST7565

Version 1.12

Support for Teensy 3.0

Version 1.11

Support for Arduino 1.0.3

Fix French keyboard layout

Version 1.10

Support for Arduino 1.0.2

Fix error message line number bug.

Add Teensy 3.0 files (arm-none-eabi toolchain is not yet in the installer, use
this copy for Teensy 3.0).

Version 0.2

Version 0.1

All: If the serial monitor is opened immediately after
an upload, the Teensy may not have been detected by the operating
system yet, and the serial monitor is unable to open the device.
Future versions will retry for several seconds to solve this issue.

Windows & Linux: When using the Serial Monitor with
the USB Keyboard/Mouse option, sometimes a "teensy gateway
communication error" can occur. Close and reopen the serial
monitor to regain communication.

Windows: You may need to run the Teensyduino installer as
administrator for it to install all files. Some anti-virus
programs can also interfere with installation and must be
temporarily disabled. A
Norton AntiVirus Workaround
was sent by Carl B.

All: The Upload button can only work if your Teensy is running
a previously loaded sketch. If your board can not be rebooted
automatically, a message will ask you to press the reset button.

Mac OS X: When you run Arduino after installing Teensyduino, the
Leopard's firewall will recognize the program has changed and will
ask again if you wish to allow internet connection (Arduino checks
for updates). Teensyduino does not "phone home", but Arduino does!
TODO: is this still an issue with recent versions of Arduino??

Linux: By default, the TeensyduinoInstall.linux32
and TeensyduinoInstall.linux64 files will not have execute permission enabled
when saved by most web browsers. Use a file manager or type
"chmod 755 TeensyduinoInstall.linux32" in a terminal to make the file executable.
Then you can run it. In a terminal, type "./TeensyduinoInstall.linux32". Replace
"32" with "64" if using the 64 bit version, or "arm" if using the ARM (Raspberry Pi)
version.

Linux: Teensyduino only works with Arduino from www.arduino.cc.
The modified version provided by Ubuntu is not (yet) supported.

Linux (especially Gentoo): Arduino uses the AVR toolchain
provided by your system. As of December 2011, Gentoo's avr-gcc is horribly
broken. Some newer Fedora (avr-gcc 4.6.x) also have trouble. Faulty
AVR toolchains are a persistent problem on Linux.

Credits

The Teensyduino installer is based in part on the work of
the FLTK project (http://www.fltk.org).

Numerous files covered by various open source licenses are installed
by Teensyduino. Nearly all are provided in source code form. Please
refer to the comments within each file for copyright and licensing
information.

Files installed in compiled binary format that are based on code licensed
under the GNU General Public License have their corresponding source code
installed in a "src" directory. On Macintosh, use control-click on Arduino
and select "Show Package Contents" from the menu.